Your man might be a sex machine now, but don't be surprised if he seems to be running on fumes as he gets older. Around age 40, some men start to have a progressive drop in testosterone, and that can really stall a guy's sex drive as well as indicate and even lead to other health problems.

When men reach their fifth decade, their testosterone -- the hormone that gives them their appetite for sex -- can begin to dip by about 1% to 1.5% per year, every year. That may not sound like much, but over time, the decline can have unpleasant consequences in some men.

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Not only can this hormonal drop contribute to erectile dysfunction and ruin the mood for sex, but it can also affect your man's attitude in general -- low testosterone levels can be associated with depressed moods, low energy, motivation, and self-confidence. It can also lead to low bone density, increasing his risks of osteoporosis. Low testosterone can also lead to reduced muscle mass and increased body fat, all of which can contribute to obesity, diabetes, and metabolic syndrome.

"I look at testosterone as jet fuel," says Edmund Sabanegh, MD, chair of the urology department and director of the Center for Male Infertility at the Cleveland Clinic in Ohio. "It keeps men running. Diminished mental clarity, motivation, drive -- all of these things can be related to low testosterone."

How Low Testosterone Affects Relationships

The more time between the beginning of symptoms and diagnosis and treatment, the greater the chances that your partner's low testosterone will impact your relationship. He's frustrated that he can't get an erection and his interest in sex has gone AWOL. That may leave you worrying that he's not that into you anymore.

So, what's happening in his body can dampen both his mood and yours. As Irvine, Calif.–based sex therapist Stephanie Buehler, PsyD, puts it: "It's a point where the biological and the psychological converge."

Learning that the problem is a physical one often comes as a relief for both guy and girl, Buehler says, but by that point, some damage to the relationship may have already been done. "There's a psychological adjustment period," she says. "She can say to herself, 'It isn't me. There was something physical, and we missed it.' But for some couples the confusion, the hurt, and the angry feelings may require some repair work. Usually, that is done quickly."

Usually, but not always. The longer the problem has been building up, Buehler says, the longer it can take to figure out and address. Typically, she works with couples for four to six sessions to work out any lingering kinks in their relationship.